1. Field of the Invention
The present invention relates to a method and apparatus utilized in a half-duplex frequency division duplex (HD-FDD) system, and more particularly, to a method and apparatus for handling hybrid automatic repeat request (HARQ) in an HD-FDD system.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard is developed by the 3GPP as a successor of a universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provide high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
An LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques such as carrier aggregation (CA), coordinated multipoint (COMP) transmission/reception, uplink (UL) multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Machine-type communication (MTC) is one type of data communication including one or more entities not requiring human interactions. That is, the MTC refers to the concept of communication based on a network such as the existing GERAN, UMTS, long-term evolution (LTE), or the like used by a machine device instead of a mobile station (MS) used by a user. The machine device used in the MTC can be called an MTC device. There are various MTC devices such as a vending machine, a machine of measuring a water level at a dam, etc. That is, the MTC is widely applicable in various fields. The MTC device has features different from that of a typical MS. Therefore, a service optimized to the MTC may differ from a service optimized to human-to-human communication.
The MTC devices, which are regarded as low cost, small and pervasive, have drawn a lot of attention in 3GPP standard developing due to their ability to augment the usage scenarios of 3GPP standard. Their transmission and reception abilities are limited (e.g., limited bandwidth usage, limited buffer and limited number of antennas). By removing extra duplexers, the half-duplex frequency division duplex (HD-FDD) mobile devices can provide further cost reduction. This extra cost reduction is helpful for pushing the MTC markets. Therefore, specific design towards HD-FDD devices is desirable in future 3GPP releases.
Although HD-FDD devices can have further cost reduction due to removing unnecessary duplexers, they suffer from their inability of having simultaneous transmission/reception in both uplink (UL) and downlink (DL) directions. If their hybrid automatic repeat request acknowledgement and negative acknowledgement (HARQ ACK/NACK) timing related to DL transmission directly follows the legacy FDD HARQ ACK/NACK timing (i.e., an ACK/NACK is transmitted on 4 milliseconds (ms) after the detection of DL transmission), the subframe where ACK/NACK is transmitted cannot be used for another DL transmission. Because there is only one ACK/NACK transmission in the UL subframe, only half of the subframes can be used for DL transmission, which results in huge reduction of throughput. Also, the frequency resource is expected to be limited for future MTC devices (e.g., the total number of used physical resource block pairs is small). Therefore, the availability and efficiency in the usage of time resources become far important. On the other hand, UL transmission suffers from the same problem. Thus, there is a need to improve the system throughput for operations in the HD-FDD communication system.